The present invention relates to magnetic bearing devices and, more particularly, to magnetic bearing devices capable of preventing undesirable consequences caused by conical mode or parallel mode resonance which is likely to occur during acceleration of a rotor.
FIG. 9 shows a general constructional diagram of a turbomolecular pump provided with a three-axis-controlled magnetic bearing. The three-axis-controlled magnetic bearing employs a radial electromagnet 4 and an axial electromagnet 6 to keep a rotor 2 levitated by magnetic force. The revolving speed of the rotor 2 is detected by a rotational speed sensor 7. FIG. 10 is a block diagram showing an arrangement for radial position control performed in a conventional magnetic bearing device. For controlling the position of the rotor 2 in its radial direction, a displacement detector 30 comprises a radial position sensor 8 to detect the position of the rotor 2 and a displacement arithmetical circuit 10 to calculate its displacement. An output of the displacement arithmetical circuit 10 is passed through a PID control circuit 12 which has a PID control function, and is amplified in power by a power amplifier driving circuit 14 and a power amplifier 16. Then, the radial electromagnet 4 is driven to actively control the rotor 2. It is understood from the foregoing that the position control of the rotor 2 is carried out based simply on its displacement as detected by the radial position sensor 8. Since the radial position sensor 8 is mounted at a higher position than the center of gravity (not shown) of the rotor 2, it monitors displacements of an upper portion of the rotor 2. On the other hand, the displacement of a lower portion of the rotor 2, i.e., the portion lower than its center of gravity, is passively controlled by the axial electromagnet 6, an armature 18 and permanent magnets 20. Therefore, the lower portion of the rotor 2 can produce large deflecting motion which can cause the lower portion to go into contact with a touch-down bearing 22 when the rotor 2 is accelerated and its revolving speed reaches a resonant point which is characteristic of the turbomolecular pump. This deflecting motion involves simultaneous conical motions of both the upper and lower portions of the rotor 2 about its center of gravity. This phenomenon can cause wear of the bearing or temporary vibration. While the resonant point of this kind tends to vary depending on the revolving speed of the rotor 2, such deflecting motion is generally divided into motions in two directions according to the revolving speed, in which the motions in the two directions occur around a common stationary point. These motions are backward turning motion (precession) directed opposite to the revolving direction and forward turning motion (nutation) directed in the same direction as the revolving direction. Here, a point where resonance due to precession coincides with the revolving speed of the rotor 2 is defined as a first conical mode resonant point, while a point where resonance due to nutation coincides with the revolving speed of the rotor 2 is defined as a second conical mode resonant point. Besides these conical mode resonances, the turbomolecular pump can occasionally produce parallel mode resonance due to translational motion of the rotor 2, and this parallel mode resonance can also cause the rotor 2 to go into contact with the touch-down bearing 22 or temporary vibration to occur. In actuality, the conical mode resonance and parallel mode resonance occur simultaneously and they are combined in a complex manner in many cases. To avoid inconvenience which can occur at resonant points in such conical mode and parallel mode resonances, Japanese Unexamined Patent Publication No. 7-248021 discloses a method for improving stiffness in a low frequency range by connecting a plurality of band-pass filters 24 in parallel with the PID control circuit 12 as shown in FIG. 11.
Although provision of a plurality of band-pass filters 24 connected in parallel with the PID control circuit 12 will be advantageous to a certain extent in allowing the rotor 2 to exceed a conical mode resonant point, for instance, an improvement in stiffness in a much lower frequency range is desired in order to enable the rotor 2 to go beyond the resonant points in a stable manner. In this respect, a conventional circuit configuration in which the band-pass filters 24 consisting of phase lead elements alone are parallel-connected with the PID control circuit 12 is associated with the following problem as compared to a circuit configuration in which the band-pass filters 24 are series-connected with the PID control circuit 12. FIG. 12 shows Bode diagrams providing a comparison between a case where band-pass filters are series-connected with a PID control circuit and a case where the band-pass filters are parallel-connected with the PID control circuit. Solid lines in this Figure are for the configuration employing a series connection while broken lines are for the configuration employing a parallel connection. It can be seen from FIG. 12 that the parallel connection is less effective than the series connection in increasing gain or advancing phase when the same band-pass filters are used. Furthermore, analysis of transfer functions, for instance, becomes more complicated in the parallel connection compared to the series connection.